Strained engineering enables 21st century wonder materials

Robert E Simpson

SUTD researchers Robert Simpson and Zhou Xilin, together with researchers from the Institute of Photonic Sciences (IFCO) and Massachusetts Institute of Technology (MIT), are investigating ways to stack two dimensional crystals, one on top of another, and designing new artificial materials based on these stacks. Their strain engineering method opens up a wealth of new opportunities to design electronic and photonic materials with bespoke properties.

Antimony telluride, Sb₂Te₃ is a two dimensional crystal that is just 5 atoms thick. Under some conditions the compound germanium telluride, GeTe, can also be grown as a two dimensional crystal. In these structures the atoms are arranged with a honeycomb structure, as shown in Figure 1. These crystals can be stacked to create an artificial 3D crystal with properties that can be switched by disordering the GeTe two-dimensional crystal. The energy needed to disorder the GeTe crystal layers can be lowered by specially designing the GeTe— Sb₂Te₃ stacking sequence.

In a recent Nature Communications paper, the researchers describe how the disordering of the GeTe two-dimensional crystal happens in a similar way to how ice melts beneath an ice skate. The pressure of the skate’s blade helps to melt the surface atoms of the ice, thus creating a slippery surface on which the ice skate slides. The interfacial layers of GeTe, which are sandwiched between the Sb₂Te₃ layers, were engineered to disorder due to the stress and pressure imposed by the Sb₂Te₃ layers. In doing so, a disordered two dimensional layer that was 100,000 times thinner than a human hair was created. These two-dimensional disordered layers can be arranged back into a honeycomb crystal structure after heating.

These results are important because the electronic and optical properties of these materials also change when the GeTe layers disorder. In particular materials based on Sb₂Te₃ and GeTe are now being employed in next generation memories for use in smart phones. Efficient and high speed memory materials can now be design based on these stacks of two-dimensional crystals.

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